Generally, OFDM (orthogonal frequency division multiplexing) uses multi-subcarrier modulation. An orthogonal frequency division transmission system uses a CP (cyclic prefix) attached to a front of an OFDM symbol by copying a signal of a last part of the OFDM symbol to compensate for multi-path in transmitting a signal. Owing to this feature, the orthogonal frequency division transmission system has excellent performance in a multi-path and moving reception environment to be spotlighted as a high-speed mobile communication transmission system.
In a wireless communication system environment, multi-path fading takes place due to multi-path duration. A process that a receiving side recovers a transmission signal by compensating signal distortion generated from a rapid environmental variation attributed to fading is called a channel estimation. And, a pilot symbol known between transmitting and receiving sides is used for the channel estimation in general.
In a wireless communication system that uses an orthogonal frequency division transmission, there are a method of allocating training symbols to all subcarriers and a method of allocating training symbols between data subcarriers. The method of allocating training symbols to all subcarriers corresponds to a method of allocating pilot symbols across all subcarriers without data allocation like a case of preamble or midamble. And, the method of allocating training symbols between data subcarriers corresponds to a method of allocating a pilot symbol to a subcarrier between subcarriers having data allocated thereto.
The method of allocating pilot symbols between data subcarriers is able to increase a transmission quantity of data but decreases density of pilot symbols, whereby a degradation of channel estimation efficiency takes place. In a single input/output transmission method, a preamble or midamble, which is known to both a transmitting side and a receiving side and has high density of pilot symbols in general, is used for channel estimation.
A receiving side, which knows information of pilot symbol, is able to estimate a channel by parsing a received signal and is also able to precisely estimate data sent from a transmitting side by compensating an estimated channel value.
In particular, assuming that a pilot symbol sent from a transmitter, channel information undergone by a pilot symbol in transmission, a thermal noise generated from a receiver and a signal received by the receiver are P, h, n and y, respectively, ‘y’ can be expressed by Formula 1.y=h·p+n   [Formula 1]
In this case, the pilot symbol P is already known by the receiver. So, channel information (ĥ) can be estimated according to Formula 2.ĥ=y/p=h+n/p=h+{circumflex over (n)}  [Formula 2]
In this case, accuracy of a channel estimation value ĥ estimated the pilot symbol P depends on a value of {circumflex over (n)}. For an estimation of an accurate value of ĥ, {circumflex over (n)} needs to converge into 0. For this, influence of {circumflex over (n)} has to be minimized by estimating a channel using a number of pilot symbols.
In a preamble used by IEEE 802.16 OFDMA system, one of 128, 512, 1024 and 2048 is selected to be used as the number of subcarriers of a frequency. Portions of both sides of the subcarriers are used as a guard interval and the rest of the areas are used for a pilot symbol per three subcarriers as follows. FIG. 1 is a diagram of an example of allocating one pilot symbol per three subcarriers.PreambleCarrierSetn=n+3·k 
PreambleCarrierSetn: all subcarriers allocated to a specific prefix
n: number 0 . . . 2 of indexed prefix symbol carrier sets
k: consecutive indexes 0 . . . 283 of prefix subcarriers
Segment 0: Prefix carrier set 0 used (PreambleCarrierSet0)
Segment 1: Prefix carrier set 1 used (PreambleCarrierSet1)
Segment 2: Prefix carrier set 2 used (PreambleCarrierSet2)
PN series usable as pilot symbols are numerated in Table 1. PN series to be used depend on a segment number and an IDcell parameter value. Each of the defined PN series is mapped to a subcarrier in a sequence of an ascending series. In Table 1, a PN series is represented by a system of progressing by 16. To obtain a corresponding PN code value, a progressing-by-16 series is converted to a binary series Wk and the Wk is then mapped from MSB to LSB. (0 is mapped to +1 and 1 is mapped to −1. For instance, since Wk is 110000010010 . . . in a 0th segment having an index 0, a converted PN code value is −1 −1 +1 +1 +1 +1 +1 −1 +1 +1 −1 +1 . . . .)
TABLE 1In-IDSeg-dexcellmentModulated Series (progressing-by-16 system)000A6F294537B285E1844677D133E4D53CCB1F182DE00489E53E6B6E77065C7EE7D0ADBEAF110668321CBBE7F462E6C2A07E8BBDA2C7F7946D5F69E35AC8ACF7D64AB4A33C467001F3B22201C75D30B2DF72CEC9117A0BD8EAF8E0502461FC07456AC906ADE03E9B5AB5E1D3F98C6E............
A pilot symbol is a signal used to perform time or frequency synchronization between at least two or more systems in wireless communications and to estimate a time-variable channel. In case of IEEE 802.16 system, a pilot symbol is arranged to have a signal of a symmetric structure in a time domain to synchronize a transmission timing. However, the related art pilot symbol transmission method has a problem that a receiving side has difficulty in recovery when a same pilot symbol is transmitted per a transmitting antenna in a multi input-output system.
Namely, in a related art wireless communication system according to single input/output orthogonal frequency division transmission, a pilot symbol is utilized for a channel estimation by transmitting the pilot symbol. Yet, in a multi input/output orthogonal frequency division transmission, a pilot symbol is difficult to be used if it is transmitted per antenna. In particular, in case that pilot symbols are simultaneously transmitted from antennas, respectively, interference exists between the transmitted pilot symbols. So, a receiver is impossible to perform independent channel estimation on a channel path at a transmitting/receiving end. Signals of different transmitting antennas work as noise components to degrade performance of a synchronizing unit in channel estimation, signal detection, frequency offset estimation and the like.